The overarching goal of this research is to elucidate how young children internalize rules and values, and thus, develop conscience, or moral character. Conscience, a complex system encompassing moral emotions, behavioral self-regulation, moral understanding, and the moral self, is perhaps the single most powerful factor that guides prosocial, rule-abiding behavior and prevents callous, antisocial conduct. Conscience develops in an intricate interplay among child temperament, social relationships, and family ecology. Two studies of adaptive and maladaptive pathways in conscience development are proposed. Study 1 will follow the current sample of 100 low-risk families. Uniquely rich, massive, multi-method, multi- trait behavioral and report data have been collected for mothers, fathers, and children at 7, 15, 25, 36, and 52 months. Those rich developmental histories will be related to children's conscience and mental health in family, school, and peer contexts, and to biological measures. Toward that end, extensive behavioral and biological assessments are proposed at 66, 78, and 96 months. Study 2 will test, in an experimental design, effectiveness of a theory-based intervention to promote conscience development in 190 24-42-month-old children of low-income mothers, a population at risk for conduct problems. Half of the mother-child dyads will participate in a 10-week intervention, designed to increase maternal responsiveness, and thus to enhance mother-child mutually responsive orientation (MRO). MRO, in turn, will promote multiple aspects of children's conscience and socio-moral competence and prevent conduct problems. All measures will be obtained prior to, immediately after, and 6 months after the intervention, and compared to the control group. Maternal and child behavior and affect will be tracked throughout the project. Child temperament, mother personality, and ecological factors of stress and support will be examined as moderating the effectiveness of the intervention. Study 2 bridges the current basic research program with developmental psychopathology and science of prevention. In both studies, analyses elucidate causal mechanisms responsible for links among constructs (mediation) and multiple causal pathways (moderation) using structural equations modeling (SEM). This work is relevant to public health because an adaptive conscience that is an effective inner guide of conduct is critical for mental health and socio-moral competence, whereas disturbances of conscience mark antisocial disorders. By focusing on adaptive and maladaptive early pathways, we elucidate early developmental risk and protective factors and translate this knowledge into an effective intervention. Together, basic longitudinal research on a low-risk group and an intervention study in a high- risk group will inform socialization theory and highlight emotional, social, and biobehavioral processes critical for understanding and preventing antisocial conduct problems.